Method of removing acid formed during cathodic electrodip coating

ABSTRACT

A method of removing, by oxidation at the anode, the acid liberated in cathodic dip-coating in the course of the deposition of the coating film, using anodes coated with a layer of tin oxide or with a mixture of tin oxide with ruthenium or iridium oxides.

The present invention relates to a method of removing the acid liberatedfrom the electrodeposition bath in cathodic electrodeposition coatingwhile the coating film is being deposited.

BACKGROUND OF THE INVENTION

In cathodic electrodeposition coating, the substrate to be coated isimmersed in an aqueous electrodeposition bath and connected as thecathode. When a voltage is applied, a coating film is deposited on thesubstrate. The coating materials employed comprise polymers that havebeen converted by protonation to a water-dispersible form. Thisprotonation is achieved predominantly through the addition of weakorganic acids. These acids accumulate in the region of the anode if inthe course of coating deposition the cationic binder is neutralized andthe coating material that has been consumed is gradually replaced bynew, protonated coating material. In order to control the pH of theelectrodeposition bath, therefore, it is necessary to remove the acidfrom the bath. This is generally done by means of what is known as theanolyte circuit. The anolyte circuit initially requires that the anodeis separated from the remainder of the electro-deposition bath by adiaphragm or membrane. This membrane is generally an anion exchangemembrane, which permits only anions—in the present case, acidradicals—to flow toward the anode. Binders and pigments, on the otherhand, are held back. By means of the anolyte circuit, acid-enrichedelectrolyte is withdrawn from the anode compartment, discarded generallyas wastewater, and replaced by water.

In addition to the anolyte circuit, electrodeposition baths normallyinclude an ultrafiltration circuit. This circuit removes bath liquiddirectly and passes it to an ultrafiltration stage whose purpose is toseparate out solvents and other coating components of low molecular massthat accumulate in the bath. Binders and pigments, on the other hand,are retained and passed back to the bath. For further details regardingthe prior art reference may be made to the literature (e.g.“farbe+lack”, 2/1981, p. 94 ff).

For removing the acid which accumulates in the bath U.S. Pat. No.3,682,814 proposes breaking down at least part of this acid by oxidationat the anode. Auxiliary measures are considered here for effectiveimplementation of the oxidation, such as the heating of the anode zoneor the addition of a catalyst solution. It is additionally intended thatanodes by used which are coated with platinum, platinum oxide and othernoble metals, chromates, manganates, vanadates, molybdates, cobalt,nickel, chromium and oxides of these metals, and other heavy metals. Theacid to which U.S. Pat. No. 3,682,814 gives particular preference in itsprocedure is formic acid. The reason for this is that the oxidativedissociation of formic acid to carbon dioxide and water consumes twocharge units per dissociated molecule. The acid in the bath cantherefore be broken down electrochemically with a theoretical maximum of50%. Other common electrocoating bath acids require more charge unitsper molecule for their anodic oxidation to carbon dioxide and water, andtherefore have even lower theoretical maximums.

Using the process specified in U.S. Pat. No. 3,682,814 it is possibleaccording to Example 1 therein to break down about 40%—out of atheoretical maximum possible 50%—of the formic acid neutralized at theanode. A disadvantage of this process is the instability of the anodesemployed. In addition, anode and cathode are separated by a membrane,since the temperatures and pH for a particularly efficient reaction inthe anode compartment are different than those prevailing in the cathodecompartment.

DE-44 09 270 also proposes anodic oxidation of the acid employed. Asalready preferred in U.S. Pat. No. 3,682,814 the acid employed here isagain essentially—in other words, to an extent of more than 90%—formicacid. Theoretically possible anode materials specified are platinum andplatinized stainless steel electrodes, platinized titanium electrodes,platinized graphite electrodes, ruthenium-doped stainless steelelectrodes or mixed-oxide-doped electrodes made from stainless steel,titanium or graphite. Unlike U.S. Pat. No. 3,682,814, however, thispatent gives no measurement results and no numerical data for thebreakdown rates achieved.

With regard to the electrodes that can be employed in electrochemicalprocesses, German Patent 15 71 721 discloses electrodes that areemployed inter alia as anodes for the chloralkali electrolysis. To avoidlosses and improve electrode resistance, use is made of oxides ofplatinum, iridium, rhodium, palladium, ruthenium, manganese, lead,chromium, cobalt, iron, titanium, tantalum, zirconium or of silicon.Further fields of use of said electrodes are in electrodialysis andelectrodeposition coating.

German Patent 16 71 422, furthermore, discloses the use in the alkalimetal chloride electrolysis of an anode comprising a titanium core witha mixed coating covering at least part of the core surface andcomprising a material formed from ruthenium oxide and titanium oxidewhich is resistant to the electrolytes and to the electrolysis products.These anodes exhibit a substantially lower degree of overvoltage and atthe same time are dimensionally stable. Building on these properties itwas possible to develop cell constructions, such as membrane cells, andhence to improve the performance of the mercury cells and diaphragmcells known hitherto.

DE-A 34 23 605 describes composite electrodes comprising anelectroconductive polymer and, embedded partly therein, catalyticparticles (support particles with applied catalyst) and processes fortheir preparation. They can be employed, for example, as an oxygen anodein the electrolytic recovery of metals from aqueous solutions. Furtherfields of use that are specified are electrodialysis andelectrodeposition coating.

EP-B 0 296 167 likewise describes the use of comparable electrodes(referred to as dimensionally stable anodes, DSA) in cathodicelectrodeposition coating. These electrodes are neither dissolved nordestroyed in the course of electrophoretic coating under theelectrodeposition conditions assumed therein, i.e. in respect of coatingformulation, current density, pH and the destructuve influence ofchlorine.

In the field of wastewater treatment, as well, various electrodes havebeen tested and employed for the oxidative breakdown of substances. Inthe course of such tests and use it has been found, in particular, thatelectrodes with a coating of tin oxide (SnO₂) are highly effective inthe electrochemical breakdown of organic substances. These electrodeshave in particular also been tested in comparison with conventionalelectrodes such as the abovementioned DSA electrodes, for example(Stucki, Kötz, Carcer, Suter: “Electrochemical waste water treatmentusing high overvoltage anodes, Part II: Ahode performance andapplications”, Journal of Applied Electrochemistry 21 (1991), 99-104;Comnimellis: “Traitement des eaux résiduaires par voie électrochimique”,gwa 11/92, 792-797; Comninellis: “Electrochemical treatment of wastewater containing phenol”, Electrochemical Engineering and theEnvironment 92, Symposium Series No. 127, 189-201).

SUMMARY OF THE INVENTION

The present invention has now set itself the object of providing amethod of removing the acid liberated in cathodic electrodepostioncoating in the course of the deposition of the coating film whichreduces the number of rinsing procedures via the anolyte circuit thatare required to remove acid and possible breakdown products of the acidor which manages completely without the anolyte circuit.

This object has surprisingly been achieved by removing the liberatedacid, preferably formic acid, by oxidation at anodes coated with a layerof ruthenium oxide, iridium oxide or tin oxide or with a mixture ofthese oxides. With the procedure of the invention is has been possibleto break down, oxidation, more than 49% of the acid in the bath. Thiscomes close to the theoretical maximum of 50% and is a considerableimprovement on the levels of around 40% specified in U.S. Pat. No.3,682,814. Such increased efficiency makes it possible to reduce thenumber of flushing procedures required to remove the acid via theanolyte circuit. Furthermore, the electrodes employed by the inventionare more chemically stable toward the medium.

It is also possible in accordance with the invention to employ acidsother than formic acid. Such acids, however, and generally lessfavorable owing to their lower theoretical maximum capacity forelectrochemical breakdown. Lactic acid, for instance, can be broken downelectrochemically only to an extent of about 35%.

Surprisingly, it is even possible to do entirely without the anolytecircuit; in other words, the electrodes of the invention can be employeddirectly in the electrodeposition bath and it is no longer necessary toseparate the anode from the cathode compartment by a membrane. In thiscase, the invention employs additional methods to reduce the acidcontent. These methods preferably comprise conventional membranemethods. These methods preferably begin with the ultrafiltrate, sincethe latter is already devoid of the relatively high molecular massconstituents of the coating material. Examples of suitable membranemethods are methods operating by means of dialysis, osmosis, reverseosmosis, electrodialysis or a further downstream ultrafiltration.Methods of this kind are described, for example, in DE-44 09 270, EP-262419, U.S. Pat. No. 4,971,672 or U.S. Pat. No. 5,091,071.

As demonstrated by Example 2 below, it is possible under theseconditions—i.e. without the anolyte circuit—to remove about 65-70% ofthe acid from the bath. The remaining 30-35% of acid can be expelled bymeans of a membrane method, e.g. electrodialysis. The use of suchadditional measures is of course also possible in baths operating withan anolyte circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the method of the invention the electrodeposition coating bathspreferably employed are those whose binders comprise synthetic resinsthat have cationic groups. These binders are preferably protonatedreaction products of epoxide-functional synthetic resins and amines.Protonation here preferably involves formic acid, acetic acid, lacticacid and dimethylolpropionic acid. Special preference is given to theuse of formic acid. These acids are oxidized at the anode into water andcarbon dioxide.

The substrate of the electrodes of the invention can consist of metal orconductive plastic. In the case of the metals it preferably comprisestitanium, tantalum, niobium or an alloy of these metals. A suitableexample is an alloy of titanium and from 1 to 15% by weight molybdenum.A particularly preferred substrate is titanium.

The layer of ruthenium, iridium or tin oxide, or of a mixture of theseoxides, that is applied to the anodes employed in accordance with theinvention preferably has a thickness of from 0.01 to 10 μm. Aparticularly preferred range is 0.1-7 μm.

The text below describes experimental examples with the method of theinvention, demonstrating especially the improvements over the prior art.

EXAMPLE 1

Efficiency of the Anodic Breakdown of Formic Acid as a Function of theElectrode Material

In a rectangular vessel (L×W×H=20×10×20 cm) fitted with a stainlesssteel cathode, 2.5 l of an aqueous formic acid solution (approximately0.2 mol/l) were oxidized with different anode materials. The currentdensity was 5 mA/cm₂, with an electrolyte temperature of 25-30° C. Aftereach 0.2 F/mol step (based on formic acid), the acid content wasdetermined by potentiometry. After 1 F/mol the experiment wasterminated, and the acid breakdown and current yield, or efficiency, ofthe anodic oxidation of acid were determined.

Acid breakdown =(c _(o) −c _(t))/c _(o)

c _(o)=concentration of acid prior to experiment

c _(t)=concentration of acid at time t of sampling or after the end ofthe experiment

In determining the current yield (efficiency), the theoreticallyrequired charge consumption of 2 F/mol of acid is taken into account forthe conversion of the formic acid to CO₂:

Efficiency=(c _(o) −c _(t))/co * Q_(o)/Q_(t)

Q_(o)=theoretical charge consumption, i.e. 2 F/mol

Q _(t)=actual charge consumption

The anodes employed (each 10×10 cm) were as follows:

a. stainless steel (1.4401)

b. RuO₂-coated titanium plate

c. IrO₂-coated titanium plate

d. SnO₂-coated titanium plate

The table which follows shows the results of the measurements. From thistable it is evident that the anodes of the invention achieve anefficiency of from 93.7 to 98.8%, as compared with 86.9% forconventional stainless steel electrodes.

With values from 48.2 to 49.2%, the amount of acid broken downelectrochemically almost reaches the theoretical maximum of 50%.

Table for Example 1 Electrode type Stainless steel (1.440I) RuO₂ IrO₂SnO₂ Charge Acid Acid Acid Acid consump- Acid break- Acid break- Acidbreak- Acid break- tion content down Efficiency content down Efficiencycontent down Efficiency content down Efficiency [F/mol] (mol/l) % %(mol/l) % % (mol/l) % % (mol/l) % % 0.0 0.198 0.199 0.199 0.198 0.20.181 8.6 85.9 0.178 10.6 105.5 0.181 9.0 90.5 0.181 8.6 85.9 0.4 0.16516.7 83.3 0.16 19.6 98.0 0.16 19.6 98.0 0.161 18.7 93.4 0.5 0.146 26.387.5 0.141 29.1 97.2 0.139 30.2 100.5 0.14 29.3 97.6 0.8 0.127 35.9 89.60.122 38.7 96.7 0.118 40.7 101.8 0.123 37.9 94.7 1.0 0.111 43.9 87.90.103 48.2 96.5 0.101 49.2 98.5 0.102 48.5 97.0 Average efficiency 86.998.8 97.8 93.7

EXAMPLE 2

Experiment on Acid Breakdown in a Semiautomatic ElectrodepositionCoating Unit (Plate Coater)

Electrodeposition Coating Material:

A. Binder Dispersion (cf. EP 074 634, Example C, but Neutralized withFormic Acid)

The example that follows shows the preparation of a cationic resin thatis neutralized by formic acid. Bisphenol A, bisphenol A diglycidyl etherand a bisphenol A/ethylene oxide adduct are heated together and form amodified polyepoxy resin. A blocked isocyanate is added as crosslinkerto this resin. The product is then reacted with a mixture of secondaryamines. The resin is partly neutralized with formic acid and isdispersed in water.

Starting materials Parts by weight Epikote 828¹ 682.44 Bisphenol A198.36 Dianol 265² 252.70 Methyl isobutyl ketone 59.66Benzyldimethylamine 3.67 Blocked isocyanate³ 1011.28 Diketimine⁴ 65.41Methylethanolamine 59.65 1-Phenoxy-2-propanol 64.77 Formic acid 85%32.92 Emulsifier mixture⁵ 15.217 Demineralized water 3026.63 ¹Liquidepoxy resin prepared by reacting bisphenol A and epichlorohydrin, havingan epoxide equivalent weight of 188 (manufacturer: Shell Chemicals)²Ethoxylated bisphenol A having an OH number of 222 (manufacturer: Akzo)³Polyurethane crosslinker prepared from diphenylmethane diisocyanate,where of 6 mols of isocyanate 4.3 are reacted first with butyldiglycoland the remaining 1.7 mol with trimethylolpropane. The crosslinker is inthe form of an 80% strength solution in methyl isobutyl ketone andisobutanol (9:1 by weight). ⁴Diketimine formed from the reaction ofdiethylenetriamine and methyl isobutyl ketone, 75% strength in methylisobutyl ketone ⁵Mixture of 1 part of butyl glycol and 1 part of atertiary acetylene glycol (Surfynol 104, manufacturer: Air Products)

Epikote 828, bisphenol A and Dianol 265 are heated to 130° C. in areactor with nitrogen blanketing. Then 1.6 parts of thebenzyldimethylamine (catalyst) are added, the reaction mixture is heatedto 150° C. and maintained at between 150 and 190° C. for about half anhour, and then cooled to 140° C. Subsequently, the remainingbenzyldimethylamine is added and the temperature is held at 140° C.until, after about 2.5 h, an epoxide equivalent weight of 1120 isestablished. Directly thereafter, the polyurethane crosslinker is addedand the temperature is lowered to 100° C. The mixture of the secondaryamines is added subsequently, and the reaction is maintained at 115° C.for about 1 h until a viscosity of about 6 dPas is reached (50% dilutionin methoxypropanol, ICI cone and plate viscometer). Following theaddition of phenoxypropanol the resin is dispersed in the water in whichthe formic acid and emulsifier mixture have been dissolved.

The solids content after this step is 35%, and rises to 37% after thelow-boiling solvents have been stripped off. The dispersion ischaracterized by a particle size of about 150 nm.

Grinding Resin (cf. EP 505 445, Example: Grinding Resin A3)

A reactor equipped with stirrer, internal thermometer, nitrogen inletand water separator with reflux condenser is charged with 30.29 parts ofan epoxy resin based on bisphenol A and having an epoxide equivalentweight (EEW) of 188, and with 9.18 parts of bisphenol A, 7.04 parts ofdodecylphenol and 2.37 parts of butyl glycol. This initial charge isheated to 110° C., 1.7 parts of xylene are added, and the xylene isdistilled off again under a weak vacuum together with any possibletraces of water. Then 0.07 part of triphenylphosphine are added and themixture is heated to 130° C. After an exothermic heat rise to 150° C.,reaction is continued at 130° C. for 1 h. The EEW of the reactionmixture is then 860. The mixture is then cooled, during which 9.91 partsof butyl glycol and 17.88 parts of a propylene glycol diglycidyl etherwith an EEW of 333 (DER 732, Dow Chemical) are added. At 90° C., 4.23parts of 2-(2′-anilinoethoxy) ethanol and, 10 minutes later, 1.37 partsof N,N-dimethylaminopropylamine are added. After a short period ofexothermicity the reaction mixture is held at 90° C. for 2 h more untilthe viscosity remains constant, and is then diluted with 17.66 parts ofbutyl glycol. The resin has a solids content of 69.8% (measured at 130°C. for 1 h) and a viscosity of 5.5 dPas (40% strength in Solvenon PM;cone and plate viscometer at 23° C.). The base content is 0.88 meq/g ofsolid resin (meq=milliequivalent=mmol of acid or base).

C. Pigment Paste (cf. EP 505 445, Example: Pigment Paste B 3, butNeutralized with Formic Acid)

To prepare the pigment paste a premix was first formed from 34.34 partsof deionized water, 0.38 part of formic acid (85% strength) and 18.5parts of grinding resin. Then 0.5 part of carbon black, 6.75 parts ofextender (ASP 200), 37.28 parts of titanium dioxide (R 900) and 2.25parts of crosslinking catalyst (DBTO) are added and the constituents aremixed for 30 minutes in a high-speed dissolver stirrer. The mixture isthen dispersed to a Hegman fineness of less than 12 for 1 to 1.5 h in alaboratory ball mill and adjusted with further water, if necessary, tothe desired processing viscosity.

D. Electrodeposition Coating Material

For the cathodic electrodeposition coating material, 36.81 parts of thebinder dispersion A are diluted with 52.5 parts of deionized water, and10.69 parts of pigment paste C are introduced into this mixture withstirring. The coating material has a solids content of about 20% with anash content of 25%.

The plate coater, equipped with pump circulation, temperature regulationunit, an attached ultrafiltration unit, but without separate anolytecircuit, is filled with 8 l of the above-described electrodepositioncoating material.

The anode used is a titanium electrode (measuring 10×10 cm), coated withiridium oxide, which is immersed directly into the electrodepositioncoating material.

In the coater, steel panels (measuring about 10×20 cm) are coatedautomatically for 2 minutes at 280 V and at 28° C. The coat thickness isabout 20 μm. After coating, the panels are dipped in the ultrafiltratein order to rinse off adhering coating material and thereby pass it backto the dip tank.

After each 50 coated panels the CED material is analyzed and isreplenished with binder and pigment paste. The meq acid analyses showthe change in the acid content as a function of the replenishment rate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a plot of the meq acid content against the “turnover” ofthe CED bath (a turnover of 1 denotes complete replenishment of thebath). Also indicated is the change in acid content of 0 and 100% acidexpulsion.

Acid expulsion 0%=doubling of the meq acid value after 1 turnover

Acid expulsion 100=constant meq acid value

FIG. 2 shows the acid expulsion based on the meq acid value after 0.3turnover (the 0.3 turnover base was chosen since at this point in timethe establishment of equilibrium between bath and ultrafiltrate isvirtually complete).

Acid expulsion=(dtrunover×meqSo+meqSo−meqSt)/dtrunover×meqSo

where:

dturnover=actual turnover=0.3

meqSo=meq acid content after 0.3 turnover (base value, see above)

meqSt=actual meq acid content

Summary: The breakdown rate or expulsion rate, after relatively highlevels to start with (establishment of equilibrium), settles down to aconstant level of 65-70% over the period of the experiment. The balanceof 100% acid expulsion must be achieved by means of an additionalmeasure, such as electrodialysis, for example.

What is claimed is:
 1. A method of removing an acid liberated in acathodic electrodepositon coating process, the method comprisingproviding an electrodeposition bath having a cathode and an anode,wherein the anode comprises a substrate and an oxide coating layer of anoxide selected from the group consisting of tin oxide, a mixture of tinoxide and ruthenium oxide, a mixture of tin oxide and iridium oxide andmixtures thereof, and breaking down the acid by oxidation at the anode,wherein the acid is selected from the group consisting of formic acid,acetic acid, lactic acid, dimethylolpropionic aid and mixtures thereof.2. The method as claimed in claim 1, wherein said oxidation takes placein an anolyte circuit.
 3. The method of claim 1, wherein the anode andcathode are not separated from one another by a membrane in theelectrodeposition bath and wherein in addition to the anodic oxidationof the acid, a separation of acid takes place with the aid of a membranemethod.
 4. The method as claimed in claim 3, wherein the membrane methodcomprises an ultrafiltration circuit present in the electrodepositioncoating bath.
 5. The method of claim 3, wherein the membrane method isselected from the group consisting of dialysis, electrodialysis,osmosis, reverse osmosis, and one or more ultrafiltrations.
 6. Themethod of claim 1, wherein the electrodeposition coating bath employsone or more binders comprising a synthetic resin having cationic groups.7. The method of claim 6, wherein the synthetic resin comprises theprotonated reaction product of one or more epoxide-functional syntheticresin and one or more amines.
 8. The method of claim 1, wherein thesubstrate of the anode comprises metal or conductive plastic.
 9. Themethod of claim 8, wherein the substrate of the anode comprises a metalselected from the group consisting of titanium, tantalum, niobium,mixtures thereof and alloys of said metals.
 10. The method of claim 9,wherein the substrate of the anode comprises a titanium alloy comprising1-15% by weight molybdenum.
 11. A method according to claim 1, whereinthe efficiency of the anodic oxidation of the acid is from 93% to 99%.12. A method of removing an acid liberated in a cathodicelectrodeposition coating process, the method comprising providing anelectrodepositon bath having a cathode and an anode, wherein the anodecomprises a substrate and an oxide coating layer of an oxide selectedfrom the group consisting of tin oxide, a mixture of tin oxide andruthenium oxide, a mixture of tin oxide and iridium oxide, and mixturesthereof, and breaking down the acid by oxidation at the anode.
 13. Amethod of removing an acid liberated in a cathodic electrodepositioncoating process, the method comprising: providing an electrodepositionbath having a cathode and an anode wherein the anode comprises asubstrate and on the substrate an oxide coating layer of an oxideselected from the group consisting of tin oxide, a mixture of tin oxideand ruthenium oxide, a mixture of tin oxide and iridium oxide, andmixtures thereof, and breaking down the acid by oxidation at the anode,wherein the efficiency of anodic oxidation of the acid is greater than87%.
 14. A method of removing an acid liberated in a cathodicelectrodeposition coating process, the method comprising: providing anelectrodeposition bath having a cathode and an anode wherein the anodecomprises a substrate and on the substrate an oxide coating layercomprising tin oxide, and breaking down the acid by oxidation at theanode.
 15. A method according to claim 14, wherein the efficiency of theanodic oxidation of the acid is greater than 87%.